Anode backfill



Oct. 22, 1957 N. o. cAMPlsE ET AL ANoDE BACKFILL Filed Aug. 28, 195o4 CONVENTION/9L 50c/(FILL v METHL CAO/VATE llene/rml /NcL 11o/M5 INVENTQHS. [Vaak 0. Campwe Chmflai W mmwm Bylaw- MMV ATTORNEY United States Patent ANoDE BACKFILL Nash 0. Campise and Charles W. Scamman, Houston, Tex., assiguors to Houston Oil Field Material Company, Inc., Houston, Tex., a corporation of Delaware Application August 28, 1950, Serial No. 181,716 3 Claims. (Cl. 204-197) This invention relates to backllls for use in the galvanic protection of underground corrodible metal structures desirable when so utilizing such anodes to bury them in prepared backfills designed particularly to control the chemical nature of the anode environment and thereby maintain perfect contact of the anode with the earth.

The ideal backfill for such anodes should have the property of being wetted rapidly, easily and thoroughly by ground waters, should be capable of retaining moisture during dry spells and should maintain a substantially constant volume under varying weather conditions in order that the backfill does not swell and then shrink.

away from the anodes. In addition, the backill should be free from splitsand dry areas, should have the property of maintaining a smooth texture for even or non-localized current distribution `at the surface of the metal anode thereby providing uniform consumption of the anode during its current production. Additionally, the backill should have good electrolytic conductivity and yet should not be so soluble as to be leached away. In addition, the backll material mustnot polarize the anode unduly or form impervious coatings'thereon.

Various mixtures of clays, bentonites and calcium or sodium sulfates have been utilized as backflll materials with galvanic metal anodes andrthese mixtures do not have all of the ideal properties that a backll mixture should have. More particularly, these mixtures are disadvantageous in that they absorb water very slowly and the backlill preparations are not compact after such absorption, that is, such backll preparations are not free from splits and dry areas and do not have a smooth texture. permit a full development of galvanic current for several days after installation and many times a full development is never realized. For example, due to hydration and immense swelling of bentonite and bentonitic clays, a water block may be and many times is formed near'the Accordingly, these backill preparations do not top of the backll column and in some instances wetting to the bottom is never realized, the dry areas contributing to imperfect contact between the anode and the soil and thereby resulting in localized current output from the anode. In addition, such backill preparations shrink unduly during dry weather and thereby markedly increase the electrical resistance of the anode circuit at such times.

Accordingly, it is a prime object of our invention to provide a backfill for use as an anodic environment in cathodic protection systems which backlill is readily and thoroughly wetted in a minimum of time and which is substantially of constant volume irrespective of its moisture content.

It is still a further major object of our invention to provide such a backiill which is rapidly wetted by ground water and which provides a smooth texture to the anode surface thereby providing even or non-localized current distribution.

`There are several methods of installation which have been employed and are well known by the art. Usually the metal anode is placed in a Vertical holeand is spaced from the top, bottom and sides, the anode backlill serving the purpose of maintaining desired contact between the anode and the earth. Conventionally, the backfill is placed around the metal anode by (a) mixing the backfill in a form of slurry and pouring it into position surrounding the metal anode; (b) the powdered backill is placed dry around the anode and water is then poured on top of the backill to wet it and thereby form contact between the anode and the surrounding soil; and (c) the backiill may be packaged in a sack together with the galvanic anode, the entire unit being placed in the hole and wet by pouring water upon it.

The disadvantages of conventional backlls are `pointedly displayed by the above methods of installation in that while a slurry produces uniform initial contact with the anode, it requires mixing equipment and many of such backlills are mixed with diiculty, particularly those containing a high percentage of bentonite, which forms lumps which are cumbersome to remove. In those methods of installation in which water is poured on top of the backflll, due to hydration and immense swelling of the bentonite and bentonitic clays, a water block is formed near the top of the backll column and, in some instances, wetting to the bottom is never realized, the dry areas Vcontributing to imperfect contact between the anode and the soil thereby resulting in localized current output from the anode.

From the standpoint of installation the mixing of a slurry is less satisfactory than otherV methods because of the expense and need of equipment for such mixing. With respect to placing the backll without prior mixing with water, we have tested a number of conventional anode backll preparations to determine their relative ability of water absorption, the time required for such absorption and their compactness, that is their freedom from splits and dry areas after such absorption. The procedure of our tests was as follows: a 25 ml. graduate was filled to the 19 ml. mark with the dry backflll preparation and 10 ml. of distilled water was poured on this column of backll and the time and amount of penetration was noted. The results of these tests are embodied in the following Table I.

Patented Oct. 22, 1957` 3 TABLE I Formula Components Remarks 63% Penetration in 5 hours, Total Penetration in 24 hours, Sticky.

15.8% Penetration 4 hrs., 31.6% Penetration 24 hrs., 37% Penetration in 48 hrs.

75 parts bentonite, 25 parts Gypsum, 10 parts Texas N ative Clay.

10 parts bentonite, 5 parts Gypsum, 4 parts Sodium, Sulfate, 3 parts Fullers Split.

21% Penetration in 1 hr., 31.6% Penetration 3 hrs., 47% Penetration in 24 hrs., 63% Penetration in 48 hrs.

15.8% Penetration 5hrs., 26%

Penetration 24 hrs.

14.5% Penetration 2.75 hrs.,

Split formed.

26% Penetration 24 hrs., 31.6% Penetration in 48 hrs.

1 part Ammonium Sulfate, parts Bentonite, 5 parts Gypsum, 4 parts Sodium Sulfate.

75 parts Coarse Gypsum, 20 parts bentonite, 5 parts Sodium Sulfate.

50 parts Coarse Gypsum, 50

parts Clay.

75 parts Bentonite, 25 parts Gypsum, 1.06 parts Soda Ash, 1.2 parts Mono- Sodium Phosphate. I 9 20 parts Texas Clay, 75 parts Total Penetration m 3% Molding Plaster, 5 parts hours.v Sodium Sulfate. 10 20 parts Texas Clay, 5 parts 6% Penetration in 16 hrs.

Sodium Sulfate.

From the above table it is manifest that only two of the compounds tested were completely penetrated with water. Of these mixtures Formula 1 showed complete penetration in 24 hours and Formula 9 showed complete penetration in three and three-quarters hours. Many of the mixtures formed splits or gumbo or sticky masses.

From the above table it is manifest that the conventional backills are not satisfactory in that they do not absorb water readily and completely in a minimum of time and provide an even texture to the anode surface.

In overcoming the disadvantages of the prior and conventional backiill mixtures, we have provided a chemical reaction which unexpectedly in combination with the normal and conventional backll components, rapidly and thoroughly wets the backtill by adsorption and provides an even texture to the anode surface. The compositions of our invention consist essentially of intimate mixtures of conventional backiill materials in combination with substantially molecular proportions of a hydrated alkaline earth oxide and an alkali metal carbonate. By substantially molecular proportions of hydrated alkaline earth oxide and an alkali metal carbonate is meant a mixture consisting substantially of one mole of hydrated alkaline earth oxide for each mole of alkali metal carbonate. The mixture may or may not include water of crystallization and the molecular mixture on wetting yields an insoluble alkaline earth carbonate and water soluble alkali metal hydroxide, the latter serving the purpose of an electrolyte. The amount of mixture of hydrated alkaline earth oxide and alkali metal carbonate in molecular proportions used in combination with the conventional backtill materials depends upon the materials utilized and also upon the resistivity of the soil adjoining the metal anode and We have found that a fraction of from about one percent to about ten percent or more of the final hydroxide concentration is satisfactory for our purpose. Holloway, on page 117, Cathodic Protections Symposium, 1949, published by the National Association of Corrosion Engineers in an article by H. W. Walquist and H. M. Fanett, indicates that a Water soluble alkali metal hydroxide increases the current output eiciency of an anode.

Accordingly, it is a still further and major object of our invention to provide a backfll mixture consisting essentially of conventional backfll materials in combination with a mixture of ingredients in molecular proportions whereby to-tal and rapid penetration may be obtained by a chemical reaction.

It is a still further and important object of our invention to provide a backll mixture which increases the current output eciency of the anode in addition to providing a non-localized current output thereof.

It is a feature of our invention that the Water soluble alkali metal hydroxide is added after installation of the backll when the material is wet by water thereby avoiding the dangers and expense attendant the handling of highly corrosive substances, such as caustic soda.

Our invention is particularly adapted for use with buried pipe lines and, accordingly, an example of the invention for the purpose of disclosure will be explained with reference thereto and with further reference to the accompanying drawing, in which like reference characters designate like parts throughout the several views, and in which:

Fig. 1 is a schematic vertical section showing one manner of using our backiill in the cathodic protection of a buried pipe line, and

Fig. 2 is a vertical section through a packaged backlill containing an anode.

The compositions of the invention consist essentially of intimate mixtures of conventional backiill materials with a mixture in molecular proportions of hydrated alkaline earth oxide and alkali metal carbonate. Of the conventional backiill materials used these materials may be either ground finely, such as a powder grind, or may be relatively coarse. That is, the conventional grinds and backll materials known to the art are operable in our invention. In addition, some inert substance, such as sand or the like, may be incorporated in our new backfill compositions; however, ysuch fillers are not necessary and may be omitted.

The following examples of our invention illustrated for the purpose of disclosure have been found satisfactory in which molecular proportions of alkaline earth oxide and an alkali metal carbonate were added to the conventional backfill materials. The test procedures used in the following examples were the same as those used in connection with Table I.

Example 1 To 7.5 parts of bentonite and 2.5 parts of gypsum, 1.06 parts of soda ash and .74 part of lime were added. A total penetration was obtained in one hour and the backll mixture had a smooth texture.

Example 2 To l0 parts of bentonite and 5 parts of gypsum, including 4 parts of sodium sulfate, 2.12 parts of soda ash and 1.48 parts of lime were added. This backfill composition showed a total penetration in one hour with a smooth texture.

Example 3 To 20 parts of Texas clay and 75 parts of molding plaster, 2.12 parts of soda ash and 1.48 parts of lime were added. A total penetration was produced in 25 minutes, the composition having a smooth texture.

Example 4 As a further example of our invention a composition comprising essentially 75 percent molding plaster, 17.4% bentonite, 4.3% barium hydroxide and 3.3% potassium carbonate showed va total penetration in one and one-half hours with a smooth texture.

Example 5 As a still further example of our invention a composition consisting essentially of 30 parts molding plaster, 7 parts bentonite, 1.98 parts potassium carbonate and 1.48 parts calcium hydroxide showed a total penetration in 3l minutes with a smooth texture.

It should be noted that in all tests the results were independent of the mesh size of the particles and that while mixtures in molecular proportions of the hydrated alkaline earth oxide and alkali metal carbonate were used, any mixture thereof may be used, the excess of either being soluble and providing no subversive effects. However, substantially. molecularproportions are preferable.

The method of using our new backiill is illustrated in Fig. 1 in which 10 is a hole in the earth that is dug to receive the anode 12, the latter being generally cylindrical. Conventionally, a central iron core 14 is provided for the vanode 12 to which is secured the electrical conductor 16 connected to the buried steel pipe 1S which is desired to be protected against corrosion. Our novel backlill composition 20 is disposed about the anode 12 in the earth cavity and the backflll is thoroughly wetted down. As indicated heretofore total penetration is produced rapidly and thoroughly and an even texture is provided by means of the chemical reaction of the molecular mixture of hydrated alkaline earth oxide and alkali metal hydroxide, the bed of backll being in rm contact with the earth and the anode.

In making the installation the cavity 10 is dug and the backll is tamped around the anode 12, the electrical conductor 16 then being secured to the anode. Ordinarily, the backiill is then wetted thoroughly. Thus an electric current l'lows from the anode 12 in the conductor l to the steel pipe 18 thereby rendering the latter cathodic and protecting it against corrosion.

An optional manner of installing the backll and anode is illustrated in Fig. 2, which is in all respects similar to that of Fig. l except that the anode and backfill are placed in a water permeable paper carton, the entire unit being placed in a cavity in the earth dug for the purpose.

In eld use of our backfll compositions it is manifest y that the amount of backiill material and the relative percentage of molecular proportions of hydrated alkaline earth oxide and alkali metal hydroxide, and in addition the number and size of anodes, will depend upon the required cathodic protection of a particular pipe line or other structure and such may be determined by engineering calculations well known to the art.

While we have spoken of aluminum, magnesium and zinc anodes, any conventional galvanic anode may be utilized, including alloys thereof and the invention is applicable to protecting any underground structure of any corrodible metal cathodic to the anode used. Moreover, our invention is operable with any of the conventional backtill materials known to the art.

While we have given several examples of embodiments and uses of our invention for the purpose of'disclosure we intend to be limited only by the spirit of the` invention as dened by the appended claims.

We claim:

l. AnV anode backll for cathodic protection of buried corrodible structures comprising, a major portion of gypsum, a minor portion of bentonite and not substantially exceeding l0 percent by Weight of a mixture of hydrated alkaline earth oxide and alkali metal carbonate in substantially molecular proportions.

2. A packaged anode for use in cathodic protection of structures comprising, a Water permeable container having an anode therein provided with means for connecting an electrical conductor thereto, and an anode backiill surrounding and in intimate contact with the anode, said anode backill characterized by being rapidly and thoroughly wetted and having a substantially even texture, said anode backll consisting essentially of a mixture of hydratcd alkaline earth oxide and alkali metal carbonate in substantially molecular proportions and additional backlill material or materials not subversive of the above characteristics.

3. A cathodic protection system for underground structures comprising, an anode cathodic to the structure buried in the earth in the vicinity thereof and electrically connected thereto, said anode being surrounded by and in intimate contact with an anode backlill, said anode backi'lll characterized by being rapidly and thoroughly wetted and having a substantially even texture, said anode backll consisting essentially of a mixture of hydrated alkaline earth oxide and alkali metal carbonate in substantially molecular proportions and additional backiill material or materials not subversive of the above characteristics.

References Cited in the file of this patent UNITED STATES PATENTS Armstrong Iune l, 1920 Robinson et al Aug. 23, 1949 OTHER REFERENCES 

1. AN ANODE BACKFILL FOR CATHODIC PROTECTION OF BURIED CORRODIBLE STRUCTURES COMPRISING, A MAJOR PORTION OF GYPSUM, A MINOR PROTION OF BENTONITE AND NOT SUBSTANTIALLY EXCEEDING 10 PERCENT BY WEIGHT OF A MIXTURE OF HYDRATED ALKALINE EARTH OXIDE AND ALKALINE METAL CARBONATE IN SUBSTANTIALLY MOLCULAR PROPORTIONS. 